1. Field of the Invention
The field of the present invention is medical imaging, and in particular medical applications of Doppler mode ultrasound imaging.
2. Background
A major limitation to open- and closed-chest manipulation of an intact organ is the ability to visualize within the organ in real time. Current examples of modalities that allow real time imaging within an intact organ include fluoroscopy, computer assisted tomography, magnetic resonance imaging, and ultrasonography. Ultrasonography, or echocardiography (echo) as it applies to the ultrasonic imaging of the heart, is the most commonly applied diagnostic modality employed to acquire real time, structural images of the heart. Echo is able to acquire structural images with high spatial resolution and fidelity to accurately measure static and dynamic anatomic dimensions and configuration, and is also able to detect relative physical motion by exploiting the Doppler effect. Accordingly, echo is able to evaluate qualitative and quantitative hemodynamic flow, turbulence, and pressure. Based on a fluid's velocity, the echo image can be labeled to display a prespecified color. For example, a high velocity fluid jet associated with the narrowing of the aortic valve can be made to appear yellow or orange. Whereas, a low velocity jet associated with incompetence of the mitral valve can be made to appear blue or purple.
Despite its value in providing accurate static and dynamic structural and hemodynamic images, ultrasonography is limited in its ability to provide high precision images of certain medical devices, such as catheters, wires, or instruments. In part, this is because of the acoustic shadowing or artifacts that can be attributed to physical properties of these devices. For example, the body of a catheter within the heart is usually discernible by echo; however, identifying a specific physical location on the catheter—such as its tip—is problematic. To facilitate the precise identification of such physical attributes, attempts have been made to improve the echogenicity of the medical device, either by physically manipulating the surface characteristics of the device, or by introducing some form of contrast agent into, or around, the device, such as air.
One technique that has met with some success is the use of real-time Doppler mode ultrasound imaging (also known as B-mode ultrasound imaging). An early technique is found in U.S. Pat. No. 5,095,910, the disclosure of which is incorporated herein by reference in its entirety, which describes locating the tip of a biopsy needle through use of Doppler mode ultrasound imaging when the tip is oscillated in the longitudinal direction. Later developments include affixing a mechanical vibrator to the proximal end of a needle or cannula to provide longitudinal vibrations down the length of the shaft, such as is described in U.S. Pat. No. 5,343,865, the disclosure of which is incorporated herein by reference in its entirety. Alternatively, U.S. Pat. No. 5,329,927, the disclosure of which is incorporated herein by reference in its entirety, describes introducing transverse flexural vibrations in a biopsy needle to render the needle more visible using Doppler mode ultrasound imaging. A more recent development, described in U.S. Pat. No. 7,329,225, also incorporated by reference herein, employs a system to automatically track the tip of a shaft within a 3D ultrasound scan by identifying local maxima in the Doppler signal. However, additional benefits may still be obtained through use of Doppler mode ultrasound imaging for locating medical devices in vivo.